hfsplus: use alloc_mutex in hfsplus_sync_fs

Use alloc_mutex to protect hfsplus_sync_fs against itself and concurrent
allocations, which allows to get rid of lock_super in hfsplus.

Note that most fields in the superblock still aren't protected against
concurrent allocations, that will follow later.
Signed-off-by: Christoph Hellwig <hch@tuxera.com>

fs/hfsplus/super.c

@@ -162,7 +162,7 @@ int hfsplus_sync_fs(struct super_block *sb, int wait)
 
 	dprint(DBG_SUPER, "hfsplus_write_super\n");
 
-	lock_super(sb);
+	mutex_lock(&HFSPLUS_SB(sb).alloc_mutex);
 	sb->s_dirt = 0;
 
 	vhdr->free_blocks = cpu_to_be32(HFSPLUS_SB(sb).free_blocks);
@@ -195,7 +195,7 @@ int hfsplus_sync_fs(struct super_block *sb, int wait)
 		}
 		HFSPLUS_SB(sb).flags &= ~HFSPLUS_SB_WRITEBACKUP;
 	}
-	unlock_super(sb);
+	mutex_unlock(&HFSPLUS_SB(sb).alloc_mutex);
 	return 0;
 }